This invention relates to transmitters and transmitting antennas and to means for reducing distortion and poor frequency response characteristics caused by limited bandwidth antennas and antenna coupling networks and also to other high powered modulated wave equipment.
Because of the limited bandwidth of some antennas, the real and imaginary components of the impedance that the transmitter sees are not constant and not symmetrical as a function of frequency. Thus, the impedance versus frequency characteristics of such electrical systems has both symmetrical and non-symmetrical impedance vs. frequency variable characteristics. This causes impedance mismatching at certain frequencies covered by the various components of the modulated wave. In many situations the higher modulation frequency components are greatly attenuated because of variations in impedance and transfer characteristics of the antenna and the antenna matching network.
Furthermore, in most cases the antenna impedance is not a symmetrical function relative to the carrier frequency. That is, the impedance above the carrier frequency is not a mirror image of the impedance below the carrier frequency. Nor does the impedance characteristic have a constant phase slope which is a condition required for constant time delay. Because of this nonsymmetry, harmonic distortion as well as frequency distortion problems are introduced. An extreme example of this would be if one sideband was shifted by 180 degrees from its original phase and the other sideband was not shifted, the amplitude modulated wave would be converted to a quadrature modulated wave with complete loss of the fundamental component and where only second harmonic components and other even harmonics would be present. This, of course, would be a situation where the system would be completely distorted and unuseable for conventional AM broadcasting.
While practical antennas do not introduce such severe problems they do degrade frequency response and introduce noticeable harmonic distortion. The present invention greatly reduces the problem so as to create a flatter frequency response and decreases envelope harmonic distortion.
Papers have been published in engineering journals describing and analyzing the problems caused by limited bandwidth antennas and the use of special networks in the antenna coupling means for improving the bandwidth of the antennas and reducing asymmetrical impedance characteristics.
For example, Mr. W. H. Doherty in the July 1949 Proceedings of the I.R.E., pages 729-734, published a paper entitled "Operation of AM Broadcast Transmitters into Sharply Tuned Antenna Systems" describing the deleterious effects of limited bandwidth antennas and describing the distortion produced by nonsymmetrical amplitude and phase-frequency characteristics of antennas. Mr. Doherty also described means for equalizing antenna system frequency characteristics using special matching networks. Other authors have published details on similar approaches to solving this problem (see Radio Engineering Handbook, K. Henney, Editor, McGraw-Hill, New York, 1959, 5th Edition, pages 20-37, 20-38 and the bibliography at 20-125 to 20-134, especially 20-128 to 20-129 covering M-f antennas). Unfortunately, these methods are expensive to utilize and the equipment is quite large and in some instances occupies more space than the associated transmitter.
U.S. Pat. No. 3,528,037, issued to Mr. K. Yamazaki, disclosed a method for compensating for nonlinear distortion and phase distortion by converting an amplitude modulated wave to a frequency modulated wave, compensating the wave, and finally converting the FM wave back to an amplitude modulated wave. This patent does not disclose means for altering the symmetry of the spectrum of the modulated wave so as to compensate for the nonsymmetrical treatment of sidebands around the carrrier by the antenna system and does not describe means for reducing envelope distortion caused by linear devices, such as an antenna, without introducing additional sidebands and increasing the bandwidth of the signal.
Conventional feedback techniques which utilize envelope detection are capable of removing envelope distortion but do not provide a satisfactory solution to the problem of limited bandwidth antennas. The reason for this is that the distortion produced due to the narrow band nature of the antenna is not produced by amplitude nonlinearities and the introduction of new sideband components, but in actuality is caused by the shift in phase or magnitude of the desired sideband and carrier components. The envelope detector feedback arrangement will introduce additional sideband components in an attempt to remove the envelope distortion with the result that the spectrum will be widened increasing interference.